Numerous references teach using mixed matrix membranes which comprise a continuous polymer phase carrier with molecular sieves dispersed therein. Examples include U.S. Pat. No. 4,925,459 to Rojey et al. and U.S. Pat. No. 5,127,925 to Kulprathipanja et al. The membranes are particularly useful for separating gases from a mixture or feedstock containing at least two gas components, generally of differing effective diameters.
Membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching one component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desirable for membrane materials to have a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Under the proper conditions, the addition of molecular sieves may increase the relative effective permeability of a desirable gas component through the polymeric membrane (and/or decrease effective permeability of the other gas components), and thereby enhance the gas separation (selectivity) of the polymeric membrane material. If the selectivity is significantly improved, i.e., on the order of 10% or more, by incorporating molecular sieves into a continuous phase polymer, the mixed matrix membrane may be described as exhibiting a mixed matrix effect. A selectivity enhancement test will be described in detail below.
One common choice of molecular sieves includes zeolites. Zeolites are a group of hydrous tectosilicate minerals characterized by an aluminosilcate tetrahedral framework, ion-exchangeable large cations, and loosely held water molecules permitting reversible dehydration.
U.S. Pat. No.4,925,562 to Hennepe et al., entitled “Pervaporation Process and Membrane”, teaches incorporating zeolites into a membrane with the zeolites preferably being as hydrophobic as possible. Zeolites with a high silicon/aluminum (Si/Al) molar ratio exhibit hydrophobic behavior in that they sorb a less polar component from a mixture also including a more polar component. The hydrophobic zeolites are less likely to be fouled by water while separating gas components in a feedstock than are more hydrophilic zeolites.
The preferred zeolites of Hennepe et al. have a high Si/Al ratio, and more particularly, have a Si/Al molar ratio of 12 or more and a silica-to-alumina (SiO2/Al2O3) molar ratio of 35 or more. These ratios can be determined by known processes, such as atomic absorption spectroscopy (AAS), X-ray spectroscopy and classical techniques, such as volumetric and titration methods. Hennepe et al. describe using polyflinctional organosilicon compounds to provide a desired interfacial adhesion between zeolites and continuous phase polymers. Poor adhesion between the zeolites and the continuous phase polymer may permit gas components to pass there between without separation. Without such adhesion, mixed matrix membranes containing these high silica-to-alumina molar ratio zeolites often fail to achieve a mixed matrix effect.
Another exemplary patent which utilizes zeolite molecular sieves in a mixed matrix membrane is U.S. Pat. No. 6,508,860 to Kulkarni et al., entitled “Gas Separation Membrane with Organosilicon-Treated Molecular Sieves”. This patent is incorporated by reference herein in its entirety. Kulkarni et al. reacts a monofunctional organosilicon compound at the site of a displaceable radical with free silanol on the molecular sieve surface. This step, often referred to as “silanation” of the sieve, typically results in substitution of the displaceable radical of the compound by the silanol of the molecular sieve. A molecular sieve having been treated in this fashion is said to be “silanated”. The monofunctional organosilicon compound thus becomes chemically bonded via a single silicon atom bond site formerly occupied by a displaceable radical prior to silanation. Kulkarni et al. further suggests that these silanated mixed matrix membranes provide an improved combination of permeability and selectivity as compared to polymer only membranes and non-silanated mixed matrix membranes.
A drawback to such silanation of molecular sieves is that compounds used in silanation, such as 3-aminopropyldimethylethoxysilane, 3-isocyanopropyldimethylchlorosilane, or allyldimethylsilane are prohibitively expensive. Another drawback is that when the molecular sieves are silanated, excess silane must be removed so the silane will not block pores. Further, the silanation process is usually performed in an organic (ethanol) rather than water solution from which the silanated molecular sieves must be recovered. The silanation and recovery steps increase the time and effort needed to prepare molecular sieves for incorporation into a continuous phase polymer. Accordingly, the cost of making membranes having silanated molecular sieves can be very high and thus commercially disadvantageous.
There is a need for additional choices of molecular sieves for incorporation into polymeric carriers to create membranes exhibiting significant mixed matrix effects. Ideally these membranes should provide enhanced selectivity and permeability, foulant resistance, and durability compared to currently available membranes. Further, it is desirable to avoid the expensive and time consuming steps of silanating the molecular sieves prior to their being dispersed into a polymeric carrier. The present invention overcomes many of the aforementioned shortcomings of membranes which utilize such high silica-to-alumina molar ratio silanated zeolite molecular sieves. Furthermore, methods of making and utilizing these membranes for gas separation are also taught.